Bicycle shifting systems comprise an actuating assembly or shifter operatively connected by a control cable within a housing to a responding assembly or derailleur. Actuation of the shifter by a rider causes the derailleur to urge a drive chain between various sprockets of a freewheel at the rear derailleur or chainrings at the front derailleur thereby effecting a gear shift. The present invention is an improvement both to shifters and front derailleurs.
Bicycle shifters can broadly be classified as lever shifters and rotatable twistshifters, each offering distinct methods of shift actuation for riders. Lever shifters, generally mounted on the bicycle frame down tube, handlebar stem or the handlebar itself, are characterized by a pivoting motion during actuation. Twistshifters, on the other hand, coaxially mounted about the handlebar, are characterized by a rotating motion substantially about the central axis of the handlebar.
Lever shifters can be further classified as conventional down tube or stem mounted shifters, thumbshifters and "push-pull" or "trigger" type shifters. Conventional lever shifters are actuated by clasping the lever between the thumb and index finger and pivoting the lever forward or backward to respectively pull or release the control cable. Thumbshifters, more conveniently mounted above the handlebar near the rider's hand, are actuated by using the thumb to nudge the pivoting lever forward or backward to the desired gear. Finally, "trigger" type shifters, likewise mounted near the rider's hand but generally below the handlebar, comprise a pair of independent levers forming an acute angle about a common pivot point, one lever being pulled by the index finger to move the drive chain from a large to a smaller sprocket, the second lever being pushed by the thumb to move the chain from a small to a larger sprocket.
One common feature of all lever shifters, however, is the need for the rider to remove at least one finger from the handlebar grip during gear shifts. This can create a potentially hazardous riding situation particularly in high performance bicycling, such as off-road mountain biking.
Rotatable twistshifters eliminate this potential riding hazard by allowing the rider to maintain a full fingered grip on the handlebar during all gear shifts. Twistshifters, generally located immediately inboard of the handlebar stationary grip, are actuated by simply rotating the shifter grip surface about the handlebar to the desired gear shift position. The present invention is in part an improvement to rotational twistshifters.
As noted above, the shifter is operatively connected by a control cable to a derailleur. Typically, a mountain bike uses a triple chainring setup including small, middle and large chainrings. In operation, the shift from the middle to large chainrings requires extra shifting force at the front derailleur, and in turn extra input torque by the rider at the shifter to effect a gear shift as compared to a gear shift from the small to middle chainrings. This extra shifting effort can be eliminated by including a mechanism in the shifter or on the front derailleur itself that discontinuously varies the mechanical advantage or "leverage" of the shifting system, thereby making the shift from the middle to large chainrings as effortless for the rider as the shift from the small to middle chainrings.
Known prior art brake lever systems incorporate similar leverage varying mechanisms to generate an increased brake cable force or braking power. A conventional brake lever assembly consists of a brake lever and a transverse mounting arm. The mounting arm is pivotally connected to a mounting bracket, the mounting bracket fixedly secured to a bicycle handlebar. The brake cable is attached to the mounting arm a select distance from the pivot point known as the "pivot arm." As the lever is squeezed by a rider, the mounting arm pivots, increasing the tension on the brake cable, thereby actuating the brake mechanism.
Said leverage varying brake lever systems, however, provide a slot in the mounting arm whereby the point of attachment of the brake cable is allowed to transition between the slot end locations which are at different distances from the pivot point. During the initial deflection of the brake lever, the cable attachment point is furthest from the pivot point, thereby providing minimal mechanical advantage. As the brake lever is further rotated, the cable attachment point slides along the slot toward the pivot point, thereby increasing the mechanical advantage and the overall braking power.
Although the noted braking device provides increased mechanical advantage, it suffers from several drawbacks. The most significant of these is the abrupt transition in mechanical advantage and hence, the force applied to the brake cable as the cable end is suddenly shifted along the slot. This abrupt increase in the cable force and resulting increase in clamping force at the wheel produces undesirable and, in many instances, hazardous braking characteristics. Accordingly, improved brake lever systems eliminate this braking hazard by altering the brake lever configuration and employing a fixed cable attachment point to achieve a substantially uniformly varying mechanical advantage without the use of slotted mechanisms.
Whereas, the brake lever systems have "taught away" from the use of abruptly varying or discontinuous mechanical advantage actuation systems to avoid hazardous braking conditions, the present invention specifically employs an abruptly varying mechanical advantage actuation system to reduce the input torque required to effect gear shifts from the middle to large chainrings.